1. Field of the Invention
This invention relates to methods and apparatus for preparing gas mixtures of predetermined composition.
2. Use and Preparation of Gas Mixtures
Accurately determined gas mixtures (the term gas as used herein includes vapors) are in widespread use as calibration or reference standards for analytical instrumentation and as feed stock for scaled down chemical reactions or processes, particularly in research and development. Thus, there is a need for mixtures of component gases in precisely predetermined proportions, and a corresponding need for methods and apparatus for the accurate production of such gaseous blends.
Commercial bottled gas and vapor mixtures of known composition are widely available for use in analytical instrumentation. The proportion of each gas component in a bottle is usually fixed when the gas is pressurized at the bottling plant, but for certain applications it is convenient for a user to mix specified quantities of two or more bottled gases to obtain a gas mixture having a specified predetermined composition different from either of the bottled gases. This composition is preferably variable by the user according to local need.
Currently available methods for the local preparation of gas mixtures by a user incorporate mass flow and permeation techniques. These methods typically use dynamic devices in which blending occurs only when gas components are flowing. These techniques are relatively inaccurate and lend themselves to applications requiring few gas components in the mixture. Indeed, some of the devices are only effective for producing specific mixtures. An example of this technique is disclosed in U.S. Pat. No. 3,948,281, incorporated herein by reference.
In contrast to the above local methods, commercial preparation of gas mixtures is usually by a gravimetric technique, in which a gas-containing vessel is weighted, or by a partial pressure technique. For several reasons, neither of these methods is adequate for the preparation of calibration gas blends suitable as reference standards for modern analytical instruments. The accuracy of the gravimetric method, for example, is dependent to a large degree on the weight of each component relative to the total weight of the vessel and its contents. This results in lower accuracies being attained in low density mixtures, such as hydrogen and helium, and also in situations where the components of interest are in low concentration.
The partial pressure method also has limited accuracy because the high pressures required to make the process commercially feasible can compound errors caused either by the absence of precise temperature control during compression, expansion and blending of the gases, or inhomogenities in the resulting intermediate and final gas mixtures. Consequently, the partial pressure method of blending gases has not been found to be particularly suitable for the preparation of accurately measured gas blends, as for calibration gases.
A survey of topics related to production of gas mixtures is presented in the 1975 copyrighted book Gas Mixtures--Facts and Fables by Frank Scarporoicer, available from Matheson Gas Products Company, 932 Paterson Plank Road, P.O. Box 85, East Rutherford, N.J. 07073; the book is hereby incorporated by reference for all purposes herein.